A magnetic induction impedance measurement (MIT) technique is usable, for example, in clinical applications, in order to determine a physiological parameter of a person. This contactless examining technique is based on measuring a signal indicative of an electrical conductivity of a tissue of the person to be examined. To this end, a respective coil arrangement for a magnetic induction impedance measurement apparatus comprises an excitation coil and a detection coil. In operation of the magnetic induction impedance measurement apparatus, a time-varying current is induced in the excitation coil such that the excitation coil generates a magnetic excitation field which penetrates through the tissue to be examined and accordingly induces an eddy current within the electrically conductive tissue of the person to be examined. The magnitude of the eddy current is based on the magnetic flux density and the connectivity of the tissue. A magnetic response field is generated by the eddy current and is detected by the detection coil in that a current or a voltage is induced in the detection coil. The current or voltage induced in the detection coil is a measure for the electrical conductivity of the tissue to be examined and depends on the conductivity and the geometry of the tissue and the geometries of the excitation coil and the detection coil.
Typically, a value of the magnetic excitation field is orders of magnitude higher than a value of the magnetic response field such that the current induced in the detection coil in response to the magnetic response field comprises a low magnitude.
Usually, the excitation coil and the detection coil of the coil arrangement are located close to one another such that the magnetic excitation field may also be present at the location of the detection coil. Accordingly, a current or a voltage may also be induced in the detection coil in response to the magnetic excitation field penetrating through the detection coil. This current or voltage may superimpose the current or voltage induced in response to the magnetic response field, whereby the detection of the magnetic response field by the detection coil may be hampered.
U.S. Pat. No. 7,164,941 B2 describes a method of and an apparatus for determining sleep states of a person. The apparatus utilizes the magnetic induction impedance measurement technique and comprises an excitation coil and first and second detection coils arranged in an axial gradiometer configuration. To this end, the first and second detection coils are electrically connected in series to one another and are arranged parallel to one another. Further, the excitation coil is arranged between the first and second detection coils and is arranged parallel to each of the detection coils. A sleep state of the person to be examined is derived from the voltage induced in the first and second detection coils. Owing to the axial gradiometer configuration, an effect of the magnetic excitation field in the first and second detection coils is cancelled by currents induced in the first and second detections coils adding up to zero. Thus, the measured voltage induced in the detection coil in response to the magnetic response field is indicative of the sleep state of the person.
However, this cancellation of the magnetic excitation field in the detection coil may be highly sensitive to mechanical misalignments of the arrangement of the excitation coil and the first and second detection coils relative to one another. Further, mechanical vibrations of the components of the coil arrangement as well as thermal expansions of the material of the excitation coil and the first and second detection coils may also reduce the cancellation of the effect of the magnetic excitation field at the first and second detection coils. Thus, an accuracy of the determination of the sleep state of the person to be examined may be poor.